Replace double spaces with hard tabs in snippets

Vim will automatically convert tabs to spaces if the settings are set
that way. Therefore, I want to use hard tabs in my snippets, so it will
always follow my configuration.
Add spec snippets

These snippets will make working with specs a little more pleasant.
Borrowed from Wincent[1].

[1]: https://github.com/wincent/wincent/blob/master/.vim/UltiSnips/spec.snippets
Improve JS snippets

I had added a "fun" snippet that duplicated the default snippet, so I am
removing it here.

The "for" snippet was made to make better use of default content reusing
information from the placeholders.